There is the conventionally known digital watermarking technology of embedding transmission data, e.g., copyright information in an acoustic signal, e.g., music or voice, with little effect on its acoustic quality (for example, reference should be made to Non-patent Document 1 or 2 below).
A variety of techniques are known as this digital watermarking technology and, for instance, Non-patent Document 1 describes the digital watermarking technique making use of such a human auditory characteristic that it is hard for a man to perceive a short echo component (reflected sound). Another known technique is the digital watermarking technique making use of such a human auditory characteristic that the human auditory sense is relatively imperceptive to change in phase.
The above-described digital watermarking techniques making use of the human auditory characteristics are effective in cases where the transmission data is embedded in the acoustic signal and where the signal is transmitted through a wire communication line. It is, however, difficult to apply the foregoing digital watermarking techniques to cases where the acoustic signal with the transmission data embedded therein is propagated through the air, for example, from a speaker to a microphone. It is because the echo component and phase in the foregoing digital watermarking techniques undergo various changes depending upon the mechanical characteristics of each of the speaker and the microphone and the aerial propagation characteristics.
On the other hand, a known digital watermarking technique effective to aerial propagation of the acoustic signal is a system using the spread spectrum as described in Non-patent Document 2 and Patent Document 1. In this system using the spread spectrum, the transmission data multiplied by a predetermined spread code sequence is embedded in the acoustic signal and the signal is transmitted to a receiver.    “Non-patent document 1” is “Echo Hiding” in Information Hiding, by D. Gruhl, A. Lu and W. Bender, pp. 295-315, 1996.    “Non-patent document 2” is “Digital watermarks for audio signals” by L. Boney, A. H. Tewfik and K. N. Hamdy, IEEE Intl. Conf. on Multimedia Computing and Systems, pp. 473-480, 1996.    “Patent document 1” is International Publication Number WO 02/45286.